Fully integrated charge pumps are used in a wide variety of applications, such as flash memories, dedicated display drivers and DC bias voltage generation for condenser microphones.
Condenser microphones such as single or double backplate micro-electro-mechanical systems devices (MEMS devices) need a high voltage charge pump. In integrated circuit designs of today, the charge pump is typically operated by a set of non-overlapping clock signals. Charge pumps for bias voltages for MEMS microphones build up their final voltage by charge accumulation on capacitors via diodes under the control of two or more non-overlapping clocks. This results in a voltage ripple on an output voltage which needs to be reduced to a level that will not introduce additional noise or deteriorate an overall power supply noise rejection (PSR) when interfacing the microphone, such as the MEMS microphone, to a preamplifier.
The voltage ripple may be reduced by filtering. For the filtering a low pass RC-filter may be used. Since there is a direct coupling from the charge pump to an input of the preamplifier of the condenser microphone, a high impedance is required to obtain a sufficient signal-to-noise ratio (SNR).
As the resistance values and the capacity values might be very large for integration, today, most filtering is based on poly-diodes in a back-to-back, anti-parallel, configuration.
US 2014/0003609 A1 shows a circuit arrangement for driving a MEMS microphone. The circuit arrangement comprises a charge pump. The charge pump is interconnected with the MEMS microphone via a first path. In the first path two oppositely connected diodes D1 and D2 are arranged. The first path splits into two parallel sub-paths UP1 and UP2, wherein in each case one of the two oppositely connected diodes D1 and D2 is arranged in each sub-path UP1 and UP2. These two sub-paths UP1, UP2 form a high impedance element.
However, the process variation and temperature variation of these filter devices based on poly-diodes is very high. In particular, poly-diodes-based circuits are often not sufficiently reliable over a required temperature range and have a wide range of parameter variation due to process variation.